Optical materials and optical products are useful to control the flow and intensity of light. Examples of useful optical products include optical lenses such as Fresnel lenses, optical light fibers, light tubes, optical films including totally internal reflecting films, retroreflective sheeting, and microreplicated products such as brightness enhancing films (BEF) and security products. Brightness enhancement films are very useful in many of today's electronic products to increase the brightness of backlit flat panel displays such as liquid crystal displays (LCDs), electroluminescent panels, laptop computer displays, word processors, desktop monitors, televisions, video cameras, and automotive and avionic displays, among others.
With respect specifically to brightness enhancement films, the index of refraction of the material composing the surface features is related to the brightness gain produced by the brightness enhancement film. Gain is a measure of the improvement in brightness of a display due to the brightness enhancement film and is a property of the optical material, and also of the geometry of the brightness enhancement film. A high gain is desired for a brightness enhancement film because improved gain provides an effective increase in the brightness of a backlit display.
Improved brightness means that the electronic product can operate more efficiently by using less power to light the display. Reduced power consumption translates into reduced heat generation and therefore means increased component life. Thus, because of these advantages, there exists a continuing need to find optical products exhibiting improved index of refraction values in the optical features.
Optical products can be prepared from high index of refraction materials, including monomers such as high index of refraction (meth)acrylate monomers, halogenated monomers, and other such high index of refraction monomers as are known in the art.
U.S. published application 2002/0123589 (Olson et al.) discloses a high index of refraction polymerizable composition for use in optical films. While this does deliver a high index of refraction optical element, UV curing is limited in size and properties of surface features it can produce. UV curing is limited in the depth or height of the feature it can produce, limiting its usefulness in some optical applications. Furthermore, UV curing cannot use the range of polymers and therefore polymer properties (such as modulus, scratch resistance) that other processes can, such as thermoplastic casting and embossing.
Polymers may be used in optical films with refractive indices less than that required for certain optical applications if modified by partial substitution of atoms, such as bromide or sulfur, for hydrogen or oxygen in the polymers. Such substituted polymers typically have increased refractive indexes but are often undesirably colored and lack thermal and photochemical stability. Therefore, these substituted polymers may not be the best choice for certain optical applications.
An alternative method of increasing the refractive indexes of polymers is combining nanoparticles with polymers. The refractive indexes of polymer/nanoparticle blends are, in part, dependent upon the refractive indices of the nanoparticles added to the polymer matrix. The theoretical refractive index of a polymer/nanoparticle blend is the volume weighted average of the refractive indexes of the nanoparticles and the polymer matrix. Consequently, it is desirable to make blends using metal oxide particles having high refractive indexes. However, metal oxide particles that are formed in water are difficult to transfer into organic liquids without particle agglomeration and concomitant transfer of water.
U.S. Pat. No. 6,329,058 (Arney et al.) discloses nano-sized metal oxide particles in a polymer matrix to form transparent blends and a method for making them. While the patent claims that the polymer of the metal oxide/polymer blend is any curable material, the examples and patent teach to the use of UV curable polymers and not thermoplastic materials. The preparation of nano-particle/polymer blends in a UV curable system is different than a thermoplastic system. The dispersants taught in the patent would most likely degrade at the temperatures used in thermoplastic extruding causing agglomeration of the nanoparticles and decomposition of the dispersants. Furthermore, the nanoparticles taught in the patent were produced in solvent systems, and then dispersed into UV chemistry and crosslinked to form the blend. Thermoplastic extrusion requires dry polymer and therefore the nanoparticles would have to be dried and re-dispersed in a molten polymer, very different than the process taught by '058. While the patent discloses the metal oxide/polymer blend in surface structures, the preferred range of 1/10th to 10 millimeters is an order of magnitude larger than the desired range for an optical film with surface features of the present invention.